Strain sensors are devices that measure strain in an object subjected to external forces. For some relatively large structures, it may be desirable to array several strain sensors across the body of the structure.
Conventional, commercially available strain gauge sensors typically use metal foil sensing elements on flexible substrates that are suitable for large-scale manufacturing. In use, these sensors are attached to an object, and as the object is deformed, the foil element of the sensor is deformed, causing a change in its electrical resistance. This resistance change is related to the strain of the object by the quantity known as the gauge factor. The gauge factor of these conventional sensors, however, is generally limited to about 2. The gauge factor of strain sensors is an index of the strain sensitivity of the gauge. The higher the gauge factor, the more sensitive the gauge and the greater the electrical output for indication or recording purposes. It would be desirable to provide strain gauges with greater gauge factor and sensitivity.
Individual nanotube-based strain sensors have been made that have gauge factors of up to 2,900. However, these are not readily scalable for mass production, and their usefulness is substantially limited by the nanoscale size of the sensing element. Incorporating these nanoscale devices into engineering applications to take full advantage of the intrinsic piezoresistive properties of carbon nanotubes (CNTs) remains a challenge. High precision fabrication processes are needed to make these sensors, and the processes do not readily lend themselves to scale up for mass manufacture of the devices.
Nanocomposite-based strain sensors also have been made. One approach includes dispersing nanotubes into a resin matrix to form electrically conductive composites. Gauge factors of up to 22.4 have been reported for these types of nanocomposite sensors. However, the sensitivity is highly dependent on the percolation threshold, which is mainly determined by the nanotube aspect ratio and dispersion. Achieving the uniformity needed for sensing elements when mixing CNTs with resin remains a challenge, because of the tendency of CNTs to aggregate and because of poor interfacial bonding between nanotubes and the resin. Accordingly, the manufacture of these sensors requires dispersion of the nanotubes around the percolation threshold, and is thus unsuitable for scalable manufacturing.
In terms of the potential for scalable manufacturability, free standing carbon nanotube networks (CNTNs) formed through either filtration/evaporation of nanotube suspension or condensing of nanotube aero-gel is thus more desirable serving the purpose of macroscopic sensing elements. However, since these networks are typically packed at the densities far exceeding the percolation threshold, the sensitivity is usually inferior to their percolating counterparts. The gauge factors of CNTN-based strain sensors were reported to be up to 7 for single-walled carbon nanotube (SWCNT) networks, 8 and 3.76 for multi-walled carbon nanotube (MWCNT) networks in small strain ranges. For large strain ranges, layers of CNTNs were laid up side by side to form a layered structure that was reported to sense strains up to 280% at the gauge factor of 0.82. Nanotube yarns formed with twisted MWCNT networks were reported to exhibit even more reduced gauge factor of ˜0.5.
It therefore would be desirable to provide improved strain sensors that overcome one or more of the foregoing limitations.